Patent GB-758230-A (GB)
Title: Improvements relating to vapour generating and vapour heating units and to a method of operation thereof
IPC: F22B3/00
Filed: 1954-08-23
Abstract: 758,230. Controlling superheat in steam boilers. BAILEY METERS & CONTROLS, Ltd. Aug. 23, 1954 [Aug. 24, 1953], No. 24427/54. Class 123(3). [Also in Group XXIX] A method of operating a vapour generating, superheating, and reheating unit provided with three separate parallel gas passes, SH, RH, BP, whereof the first two house the convection superheater 7 and the convection reheater 8 whilst the third constitutes a by-pass to the other two, comprises maintaining a pre-determined relationship between the final temperatures of superheat and reheat by proportioning the gases between the first two passes by means of dampers 9, 10, and limiting the sum or average of the final temperatures of superheat and reheat by control of the by-pass in dependence on that sum or average by a damper 11. In carrying out the invention, the dampers in the three passes may be actuated by electric motors under the control of an attendant who can observe meters giving indications of the variables involved. In an automatic, pneumatically-operated control system for carrying out the invention, the temperatures of superheat and reheat are measured separately in a controller 30 and caused to actuate pilot valves 31, 33, as disclosed in Specification 425,507 [Group XII], to give output pressures in lines 32, 34 representative respectively of the superheat and reheat temperatures. These two pressures are applied to, and compared in, a relay 35, similar to that disclosed in Specification 461,389 [Group XXIX] and the ouput pressure of this relay is applied through an automatic-manual selector 38 to motors 45, 48 which operate, respectively, the superheater and reheater dampers 9, 10. Differential operation of the two sets of dampers is obtained by interposing a reversing relay 39 between selector 38 and superheater dampers 9. The ouput from relay 35 provides a positioning control of low sensitivity and an integral control of high sensitivity. The pressures actually controlling motors 45, 48 are derived from relays 41, 42 in the output of which the magnitude of the effect of the loading pressure applied to chambers A, C is twice that applied to chamber B. Further automatic-manual selectors 44, 47 are interposed between the motors and relays 41, 42. The rate of flow of combustion gases is obtained by a meter 50 which measures the pressure drop across a portion of the gas flow path, and a pilot valve 51 actuated by the meter 50, applies to a relay 53 a pressure representative of that flow. A pressure representative of the total of superheat and reheat temperatures is obtained by adding in a relay 55 the pressures from lines 32, 34, and the ouput from this last-mentioned relay is applied through a further relay 57 to relay 53 where its effect is added to that given by the measured gas flow. Relay 53 controls directly a motor 62 actuating the by-pass damper 11, and its ouput is also applied through a relay 63, in which its effect is doubled, to the relays 41, 42 where it acts in opposition to the control impulse derived from the comparison of superheat and reheat temperatures. Relay 55 may be modified by interconnecting its chambers B, D to give an effect proportional to the average of the superheat and reheat temperatures. When the optimum final temperature of superheat is attained, electrical contacts 60 in the controller 30 close and a solenoid 58 is actuated to open a bleed between the two lower chambers in relay 57 and so convert the latter from a simple relay to one giving an integral action in addition to a positional control. The solenoid 58 may be replaced by a bellows subject to a pressure representative of the total or average of the final temperatures of superheat and reheat.